A laser diode is a semiconductor diode, the p-n junction of which functions as a laser gain medium. To obtain population inversion in the gain medium, a forward current is applied to the p-n junction. Cleaved or polished facets of the laser diode chip can be used to form the laser cavity. An external mirror or a Bragg reflector can also be used for this purpose.
Laser diode chips need to be protected from dust, humidity, electrostatic discharge (ESD), and mechanical damage. Reliable electrical contacts need to be established to apply the electric current to the p-n junction, and heat generated in the p-n junction upon application of the current needs to be removed to prevent laser diode chips from overheating. To that end, laser diode chips are usually placed in metal enclosures equipped with heat sinks, electrical contacts, and windows for outputting the laser light.
It works best to include laser diode chips into individual hermetically sealable enclosures, although this latter requirement may be alleviated to some degree by applying a protective coating to laser diode chips. By way of example, Kunihara et al. in U.S. Pat. No. 6,784,511 disclose a laser diode chip encapsulated in a transparent silicone resin. Detrimentally, transparent epoxy layer effectively limits maximum amount of optical power emitted by the encapsulated laser diode chip, so the method of Kunihara et al. is only applicable to relatively low-power laser diodes. Furthermore, epoxy or silicone resin coating does not provide an adequate electromagnetic interference (EMI) protection of the laser diode chip.
A more common prior-art can-type package, which also works for laser diodes having output optical power of several hundred milliwatt, is shown in FIG. 1. A packaged laser diode 10 includes a steel header 11, on which a copper heat sink 12 is soldered. A laser diode chip 13 is soldered to a submount 14, which is then attached to the heat sink 12. A first wirebond 1 connects the top side of the laser diode chip 13 to a laser electrode 15. The bottom side of the laser diode chip 13 is electrically coupled to the header 11. A monitoring photodiode 16 is placed in the package 10 to measure optical power of light emitted by the laser diode chip 13. One terminal of the monitoring photodiode 16 is connected to a photodiode electrode 17 via a second wirebond 2. The other terminal of the monitoring photodiode 16 is electrically coupled directly to the header 11. A middle electrode 18 provides an outside electrical connection to the header 11, thus closing the electrical circuit for both the laser diode chip 13 and the photodiode 16. A hermetic cap 19 having therein a window 19A is soldered or welded to the header 11.
The packaged laser diode 10 has several drawbacks. One, perhaps unexpected, drawback is electromagnetic sensitivity. Even though the laser diode 13 is shielded by the cap 19 from outside electromagnetic fields, the direct coupling of the laser diode 13 to the header 11 can cause EMI problems, especially when the laser diode 13 is driven by pulsed current, because both the cap 19 and the header 11 can radiate electromagnetic energy when subjected to the pulsed current. Another drawback is that, due to the geometry of the packaged laser diode 10, each laser diode chip 13 has to be packaged individually. The individual packaging of the laser diode chips 13 increases manufacturing costs. Yet another drawback is a relatively large size of the packaged laser diode 10.